Conventional military fighter aircraft are powered by high performance gas turbine engines having relatively high thrust-to-weight ratio for providing high acceleration rates of the aircraft. The aircraft gas turbine engine typically includes a variable area converging-diverging exhaust nozzle at a downstream end of a conventional afterburner or augmenter. The exhaust nozzle includes primary and secondary exhaust flaps which define converging and diverging channels through which combustion gases from the engine are discharged for generating thrust.
The exhaust nozzle is conventionally positionable for generally two modes of operation: a dry engine operating condition, wherein the afterburner is deactivated, and the primary and secondary exhaust flaps are in a generally fully closed position; and a wet, or augmented operating condition wherein the afterburner is activated and burns additional fuel for providing increased thrust, and the primary and secondary flaps are in a generally fully open position. Of course, the exhaust nozzle primary and secondary flaps are also conventionally positionable at intermediate positions in each of the dry and wet modes.
A conventional military aircraft may also include an Environmental Control System (ECS) which requires extraction of engine compressor bleed-air at pressures typically at least 40 psia. Furthermore, the engine typically includes a generator requiring a minimum shaft rpm for providing electrical output power for the aircraft.
With the aircraft operating in take-off and cruise modes of operation and during dry and wet modes of operation, the engine is amply effective for providing the required ECS bleed-air as well as electrical power from the generator. Furthermore, the engine is operable in a conventional ground idle operating condition wherein the throttle is set back to a minimum thrust and power setting of the engine, which is typically less than about 6% maximum dry thrust of the engine. However, in order to obtain acceptable levels of ECS bleed-air and acceptable power from the generator, the ground idle operating condition requires a core engine speed typically of about 70% of maximum speed, although the conventional fan speed is substantially lower.
Since the engine is a high performance engine having a high thrust-to-weight ratio, this relatively high core speed results in substantial thrust from the engine during the ground idle operating condition. This thrust is typically sufficient for causing the aircraft to roll on the ground unless braking is utilized. Of course, such braking during ground idle operating condition, substantially increases wearing of the aircraft's brakes, tires and wheels. Furthermore, during icy runway and taxiway conditions, braking through the wheels is relatively ineffective for accommodating the ground idle operating condition thrust.
Yet further, these aircraft are typically operated world wide and operate on a wide variety of runways/taxiway surface conditions, including water and ice accumulation, and with varying degrees of ramp congestion of other aircraft. Under these conditions, a relatively low level of ground idle thrust is desirable for maintaining safe landing and taxiing speeds.
Accordingly, the aircraft's brakes, as above described, may be utilized for accommodating the relatively high ground idle thrust encountered during landing, taxiing, and standing, but this is generally undesirable in view of the increased wear associated therewith. Of course, the ground idle operating condition of the engine could be preselected for obtaining relatively low core engine speeds for reducing ground idle thrust from the engine. However, if the core engine speed is so reduced, acceptable ECS bleed-air and generator output will not be obtained from the engine, thus requiring an auxiliary compressor and generator. This is undesirable in view of the increased weight, cost and complexity of such systems in the aircraft.